Less than one percent of water on the earth's surface is suitable for direct consumption by human population and industries. Most of the surface fresh water is provided by rivers and lakes. Many places that do not have the geographic benefit of being close to such rivers and lakes suffer from a scarcity of fresh water. While transportation of fresh water from lakes and rivers through long pipelines, and drawing underground fresh water have been popular solutions, such resources are becoming scarcer as time goes by.
The salinity of water is usually represented by a total dissolved solid (“TDS”) count, expressed in mg/L of salts dissolved. The salinity of sea water varies between 33,000 and 37,000 mg/L, and an average of 35,000 mg/L is accepted as the TDS count for sea water. Water with greater than 2,000-3,000 TDS is generally considered too salty to drink. Brackish water has no strict definition, and its salinity, depending on its location, may be between 2,000 and 35,000 TDS. Water having salinity in the range of 500 and 1,000 TDS is considered drinkable, but often has a strong taste. Most public water supplies in the world keep the TDS count below 500 for fresh water.
Sea water contains many species of salts. On average, out of 35,000 mg/L of TDS in sea water, sodium chloride contributes 30,000 mg/L, with the remaining 5,000 mg/L being primarily calcium, potassium, magnesium, and sulfate ions.
Obtaining fresh water by desalinating brackish and sea water has been an age-old practice. Thermal distillation was the earliest method used to desalinate sea water on a commercial basis, and improved distillation methods continue to be used today. The process involves distilling saline water and condensing the water vapor to obtain fresh water. While fresh water of great purity, for example below 25 TDS may be obtained with distillation, it remains an energy-intensive process due to the large specific heat and latent heat of vaporization of water. It is therefore commercially viable in places where plenty of waste heat is available, such as near power plants; where fresh water is scarce for natural reasons, such as arid coastal regions like the Persian Gulf; or where energy is available very inexpensively, such as in the Middle-East.
Modern technologies in thermal distillation include multistage flash (“MSF”) and multi-effect distillation (“MED”). Because heat from the condensation of water vapor must be reutilized in order to make the distillation processes cost effective, thermo-mechanical arrangements are critical in such processes. Also, due to the thermal overhead, only very large distillation installations achieve good energy efficiency. In general, the energy consumption reaches 5-9 kWh/m3 in large installations.
A reverse osmosis (“RO”) process uses semi-permeable membranes and a driving hydraulic force of 150 to 1200 psi to remove dissolved solids from brackish or sea water. Under high pressure, water molecules move through the membrane whereas salt ions do so at a rate many orders of magnitude lower. Thus, the majority of dissolved salts are removed by the RO process. Generally, higher salinity requires higher pressures for desalination, and often, multiple pressure stages are employed to drive water through the RO membrane. RO is still an energy-intensive process, and typically, 3-10 kWh/m3 of energy is required for sea water desalination using the RO process. Also, because the bulk of the water flows through the membrane, particulates and other impurities in the feed water can clog and foul the membrane unless the feed water is carefully pre-treated. Other problems with the RO process include its high installation and capital cost. In addition, because of the high driving pressure on the front side of an RO membrane, and the corresponding low back pressure, it is not uncommon that more than 70% of the feed water goes into the waste stream during the RO process.
Among modern desalination technologies, electrochemical methods have been employed. One such electrochemical method is electrodialysis (“ED”), which is a voltage-driven membrane process. An electric potential is used to move salts through a membrane, leaving fresh water behind as a product. ED takes advantage of the fact that most salts dissolved in water are ions, and are either positively charged or negatively charged. Because like charges repel each other and unlike charges attract, the ions will migrate toward electrodes having an opposite electric charge. Suitable membranes can be constructed to permit selective passage of either positive or negative ions. In a saline solution, dissolved ions such as positively-charged sodium and negatively-charged chloride migrate to opposite electrodes, passing through selected membranes that allow either cations or anions, but not both, to pass through. During the ED process, the salt content of the water channel is diluted, while concentrated solutions are formed at the electrodes. In an ED unit, membranes are usually arranged in an alternating pattern, with an anion-selective membrane followed by a cation-selective membrane. Concentrated and diluted solutions are created in the spaces between the alternating membranes, and the spaces bound by two membranes are called cells. Typical ED units consist of several hundred cells bound together with electrodes, and is referred to as a stack. Feed water passes through all of the cells simultaneously to provide a continuous flow of desalinated water and a steady stream of concentrate (brine) from the stack.
A variation of ED, called an electrodialysis reversal (“EDR”) process operates on the same general principle as an ED unit, except that both the product and concentrate channels are identical in construction. At intervals of several times an hour, the polarity of the electrodes is reversed, causing ions to be attracted in the opposite direction across the membranes. Immediately following reversal, the product water is removed until the lines are flushed out and desired water quality restored. The flush takes just a few minutes before resuming water production. The reversal process is useful in breaking up and flushing out scales, slimes, and other deposits in the cells before their accumulation causes adverse effects. Flushing helps to reduce the problem of membrane fouling. Because the concentration gradient of the salts plays an important role in such a bipolar membrane structure, the voltage needed to maintain the gradient rises with the magnitude of the gradient. The voltage cannot be raised above the electrolytic decomposition voltage of water, which is about 1.3V, so the maximum salinity at which ED/EDR works is limited. As a result, this technology is normally used to desalinate brackish water, rather than high salinity water such as seawater.
Capacitive Deionization (“CD”) is an electrosorption process whereby ions are removed from saline water using an electric field gradient as the driving force. The saline feed flows through electrodes comprised of materials such as carbon-based aerogels. These aerogels have very high surface areas, typically 400-1,000 m2/g, which contributes to higher charge capacitance than a simple flat plate. A direct current is imparted, with a potential difference of 1-2 volts, and the cations are attracted to the anodic electrode, while the anions are attracted to the cathodic electrode. Ions are held at the surface of the electrode in an electric double layer. This technology can desalinate brackish water having an initial salinity of 2,000-4,000 TDS to below 500 TDS, thus making it drinkable. With good system design, typically, up to 70 percent of the stored charging energy can be recovered in the discharging process. As a result, the net energy consumption of CD is less than 0.5 kWh/m3. However, water having salinities higher than about 4,000 TDS cannot be desalinated using this process, as the concentration gradient across the capacitive field would be too large to be separated with the application of the electrostatic force.
As described, different existing desalination technologies have various benefits and shortcomings. For example, CD and ED technologies, although energy efficient, and scalable to small units commercially, are not suitable for sea water desalination. RO and thermal distillation technologies such as MED and MSF can be used for sea water desalination, but are energy demanding and do not scale commercially. To avoid these and other deficiencies, there is disclosed a novel electrochemical desalination method that is highly efficient, highly scalable, and is effective for desalinating high salinity water such as sea water.